In recent years, a resistance-change memory aiming to significantly exceed the limit on the bit density is being proposed.
For example, considerations are given to using a ternary oxide including a transition metal element such as perovskite and spinel, a binary oxide of a transition metal, etc., as a recording material.
In using these materials, in regard to writing/erasing, a low-resistance state (on) and a high-resistance state (off) can repeatedly be changed by an application of a voltage pulse. That is, by corresponding these two states to binary 0 and 1, such can be made to function as a memory for recording data.
Further, in regard to reading, it is performed by flowing a small read current that will not cause the writing/erasing in the recording material, and measuring the electrical resistance of the recording material. In general, a ratio of the resistance of a high-resistance phase and the resistance of a low-resistance phase is about 103.
What is most characteristic in this type of resistance-change material is that it is operable in principle even if an element size is reduced to about 10 nm. In this case, a bit density of about 10 terabytes per square inch (Tbpsi) can be realized.
Further, such a resistance-change memory is also useful for three-dimensional configurations.
However, in a resistance-change memory that is configured three-dimensionally as above, in accordance with an increase in a number of stacked layers, a number of times of alignment upon using a photoetching process (PEP) increases. Further, since one alignment is performed by identifying a level difference (alignment mark) of a topmost layer, a special process for constantly remaining this level difference in the topmost layer becomes necessary.